It has previously been proposed to provide an insulating cap for a distributor in which a circular internal space is defined by the cap, and in which high-voltage connecting towers are located adjacent the outer region of a roof surface of the cap. One such construction is shown, for example, in U. S. Pat. No. 4,338,895, Lennis and Handy. The cap has a circular internal chamber. The connecting towers extend from the outside and are continued in form of a ridge or bead which extends around the outer cap. The arrangement is so provided that there is sufficient clear space between the high-voltage connections of the electrodes, and the overall arrangement is designed to improve the high-voltage insulation between the electrodes, and electrode connections and connecting elements within the tower. The arrangement of the towers is suitable, although the bead which surrounds the towers on the top cover of the electrode tends to accumulate dust, dirt, and other contaminants, which detracts from resistance to spurious high-tension discharges, creep circuits and the like. When used in connection with an automobile engine, where the engine compartment is subjected to contamination by salt air, if used close to the ocean, or even salt spray, when passing, for example, through puddles of a previously salted road, contaminant may collect within the bead and further reduce resistance of the cap structure to leakage currents and the like.